Paired membrane-filament complexes in an acoustic neuroma.

An acoustic neuroma that developed in the posterior fossa of a 43-year-old black man was studied ultrastructurally. Unusual paired membrane-filament complexes were noticed in the cytoplasm of many tumor cells and appeared as multilayered parallel and concentric arrays in various planes of section. The paired membranes were found to be discontinuous with other membrane systems of the tumor cells. The cisternal space measured 25 nm across and housed a regularly arranged, single file of filaments 9 nm in diameter spaced 12 nm apart. In view of the biologic significance and possible origin of these structures, the paired membrane-filament complexes are morphologically compared with the myelin sheaths and cytoplasmic processes in this tumor.